1. Field of the Invention
This invention relates to an apparatus and method for detecting the density of logs, cants, timbers, poles or trees. More particularly, the present invention relates to the detection of anomalies such as knots in wood.
2. Discussion of the Background
Lumber is typically sorted and classified primarily on the basis of the number of knots or other defects or anomalies therein. Lumber value may be increased by theoretical detection of internal log characteristics followed by orientation of the log on the carriage. In addition, cants sawn from logs are then sent to a resaw for processing into lumber.
Bowyer and Haygreen in Forest Products and Wood Science (1982) note that the characteristics of certain abnormal wood types such as compression wood, tension wood and juvenile wood can result in inferior performance of the wood products they contain. Compression wood is formed on the ground side of boles leaning softwood trees; tension wood is formed on the side opposite the ground on leaning hardwood trees. Juvenile wood formed near tree pith in the first 10 to 15 years of tree growth. Compression wood may exhibit longitudinal shrinkage up to 10 times more than that of normal wood. Wood products containing both normal and compression wood exhibit differential shrinkage when dried. Warpage is the result of this differential shrinkage.
Tension wood also shrinks abnormally longitudinally with the same warpage problems resulting. When machined, tension wood often has a fuzzy surface making furniture finishes substandard. Cell collapse during drying also frequently occurs resulting in a lumber grade-reducing drying defect.
Juvenile wood has thinner cell walls with resultant lower strength properties. Lumber with low strength properties may be assigned to a lower lumber grade. For this reason restrictions on purchases of young timber have been imposed in recent years as plantation timber growth rates have accelerated.
Differential orientation of the log or cant can change the location of the defects in the lumber sawn from the log and thereby change lumber grade and resultant value. To date, research on log positioning has focused on angular orientation of the logs about their central axes. Most defects in both logs and lumber are knot defects so that most of the influence on value change results from changing knot location in lumber by rotating the log. Peter et al., Forest Prod. J. 2(11):47-50 (1962); Peter, Forest Prod. J. 17(11):19-24 (1967); Tsolakides, Forest Prod. J. 19(7):21-26 (1969); Wagner et al., Forest Prod. J. 25(10):24-28 (1975); Richards et al., Res. Pap. FPL-356, USDA Forest Serv., Forest Prod. Lab. (1980); Steele et al., Forest Prod. J. 44(3):69-72 (1994) have all conducted studies relating to the influence of angular orientation on increased lumber value and have indicated increases ranging from 9 percent to 21 percent for hardwood and 7.5 percent for southern yellow pine. All of these referenced studies, however, involved some form of computer simulation of the sawing process to determine increased lumber value.
Application of various technologies to perform internal log scanning have been pursued. For example, Szymani, Scanning Technology and Process Optimization: Advances in the Wood Industry, Miller Freeman Books (1999), discloses attempts at X-ray, NMR and ultrasound log scanning. A basic presumption for the application of internal log scanning to log sawing is that the knowledge of internal defects will lead to choosing the best sawing position and method and, therefore, will allow sawmills to realize potential gain. Development of devices for internal log scanning, however, requires solving numerous technical and cost problems before industrial application is feasible.
Shafer and Ross (2000) have disclosed, in U.S. Pat. No. 6,029,522, an ultrasonic device that allows detection of localized anomalies such as knots, decay and voids in logs. Multiple measurements allow generation of maps of anomaly location such that sawing decisions can position sawlines to produce lumber with maximum value. Haddox, Wood Technology 127(2):22-27 (2000), reports a commercial installation of an ultrasound cant scanner.
A number of patents and publications, for example, U.S. Pat. No. 3,549,986 to Prine, U.S. Pat. No. 4,123,702 to Kinanen et al., U.S. Pat. No. 4,500,835 to Heikkila et al., British Patent Specification No. 1,489,554 and Finnish Patent Publication No. 53,365, disclose the use of microwaves to detect knots or slope of grain. Other patents such as, for example, U.S. Pat. No. 4,972,154 to Bechtel et al., U.S. Pat. No. 3,805,156 to Norton et al. and U.S. Pat. No. 5,585,732 to Steele and Kumar, disclose devices that employ radio frequency waves to detect knots or slope of grain in lumber. Neither the microwave nor the radio frequency lumber scanning devices describe the ability to detect knots or other types of density differences in logs, cants, timber, poles or trees.
Kaestner et al., Microwave Polarimetry-Based Wood Scanning, Proceedings of the 12th International Symposium on Non-Destructive Testing of Wood, September 13-15, University of Western Hungary (2000), describe a device that employs a waveguide that both transmits microwaves and receives the reflected waves in the range of 4 to 8 GHz. The researchers employed analysis of the polarized signal because attenuated signals from surfaces within the log were so weak as to be difficult to identify. Internal knots and other density-related areas in scanned log sections were able to be delineated in the tomographic slices produced by this microwave scanning device. This Kaestner et al. device differs from the TLDD in that microwaves rather than radio frequency signals are employed to detect density-based anomalies. Wave guides are employed as sending and receiving transducers by the Kaestner et al. device while the TLDD employs electrodes as sending and receiving transducers.
A number of references disclose the use of computer tomography imaging in combination with technology such as X-ray technology to detect anomalies in logs. For example, U.S. Pat. No. 6,026,173 to Svenson et al. discloses a microwave tomographic device to detect dielectric property differences within biologic tissue. Multiple receiving and transmitting waveguides are arranged in a ring, and the object to be scanned is placed within the circular array. Multiple frequencies are transmitted through the tissue in frequencies ranging from 0.1 to 300 GHz by opposing transmitting and receiving waveguides. Signal analysis of the multiple frequency response to dielectric differences within the tissue allows imaging of the structures responsible for these differences. As with the Kaestner et al. device, the Svenson device employs microwaves transmitted and received with wave guides. This differs from the TLDD in that radio frequency signals are transmitted and received with electrodes.
Huang et al., Tomographic Imaging of Two-Component Flow using Capacitance Sensors, Institute of Physics Publishing Co. (1989), and U.S. Pat. No. 5,130,661 to Beck et al. describe the development of a laboratory prototype of a capacitance electrode scanner for performing application of Electrical Capacitance Tomography (ECT) to obtain images of two-component fluids flowing through pipes. The device consists of an eight-electrode capacitance sensor in which the 8 electrodes are arranged equidistant from each other on the external periphery of the circular pipe through which the fluid flows. The capacitance of a 2 MHz radio frequency signal is measured between all possible pairs of the 8 electrodes. An image of the respective area occupied in the internal cross section of the pipe by each component of the two-component fluid flow is developed. This is accomplished by computer reconstruction of the measured capacitance values with a linear back-projection algorithm.
Plaskowski et al. in their book Imaging Industrial Flows: Applications of Electrical Process Tomography (1995) describe ECT technology in considerable detail. This technology applies electrodes to measure the cross-sectional capacitive components of a radio frequency signal transmitted across pipe diameter or across similarly circular-shaped cross sections. The original goal of the ECT technology was to differentiate the components of two-component non-conducting fluids flowing in a pipe. The capacitor electrodes employed by ECT are relatively large in comparison to the cross sectional diameter of the scanned medium because of the low conductivity of the medium. In the 8-electrode system, described as an example, electrode length was given as 110 mm or 1.2 times the pipe diameter of about 92 mm. The capacitance electrodes are required to be of large area because of the low signal levels they are required to sense. For the described example the 8-electrode system with 110 mm wide electrodes has a minimum standing capacitance of about 0.3 pF between electrodes positioned opposite each other and separated by the pipe diameter.
Torgovnikov, in Dielectric Properties of Wood and Wood-Based Materials (1993), classifies wood as having a conductance below that of semiconductors even when it is of maximum moisture content. Therefore, wood is considered to be non-conductive.
By the ECT method a radio frequency signal is applied to a single electrode while all remaining electrodes are grounded and act as receiving electrodes. Loser et al. Meas. Sci. Technol. 12:1083-1091 (2001) describe the two alternative ECT methods for grounding and acquiring signal data from non-sending electrodes. By one method all non-sending electrodes are grounded and signal information can be acquired from all simultaneously but with discrete values obtained from each receiving electrode. By a second method, only one receiving electrode is grounded at a time and signal information is acquired at that time, as described by Loser et al., Meas. Sci. Technol. 12:1083-1091 (2001).
For the 8-electrode example, as previously described by Huang et al. and Beck et al., the total number of possible electrode combinations is 28. Neither Huang et al., Beck et al. nor Plaskowski et al. indicate that bi-directional electrode query is important to the accuracy of their apparatus.
The capacitance electrodes described by Plaskowski et al. for ECT technology are not in direct contact with the medium being scanned. Rather, the electrodes are in contact with the pipe surface and are separated from the medium by the pipe wall thickness. In this sense, these electrodes are non-contacting electrodes. The ECT electrodes are necessarily fixed in place as they must be in direct contact with either the pipe or similar sleeve surface.
The ECT devices described by Huang et al., Beck et al. and Plaskowski et al. differ markedly from the TLDD in several ways. The ECT electrodes are large relative to the diameter of the medium being scanned to increase their sensitivity. The TLDD electrodes are small in area compared to the diameter of the medium scanned. For example, TLDD electrodes may be 1xe2x80x3 long and may be separated by a diameter of scanned medium by 20xe2x80x3 or more. The capacitor-to-diameter ratio for this example is 0.05 compared to 1.2 for the ECT example given by Plaskowski et al. The ECT electrodes are non-contacting while those of the TLDD are in direct contact with the medium. By one ECT method, all electrodes other than the sending electrode are grounded; by a second ECT method only one receiving electrode is grounded at a time while other electrodes are not grounded. The receiving TLDD electrode(s) are grounded in both ECT methods described above. In addition, the TLDD receiving electrode(s) may not be grounded when measuring voltage or impedance. In addition, for ECT the electrodes are fixed while for the TLDD the electrodes may move freely as long as direct contact with medium surface is maintained. For ECT scanning the signal applied is limited to a radio frequency signal and the voltage is measured at the receiving electrode. For the TLDD, the applied signal may be a radio frequency signal, and the received signal may be measured as voltage, impedance or current. Also, by the ECT method only signal magnitude is measured while the TLDD measures both signal magnitude and phase shift. For ECT bi-directional electrode sensing is not an issue while for the TLDD, when voltage is measured at the receiving electrode, this bi-directional sensing is important in locating anomalies and determining their size and location. Therefore, for TLDD voltage measurements the total number of electrode pair combinations is double that for ECT because bi-directional electrode query is employed.
Plaskowski et al. (1995) describe technology termed Electrical Impedance Tomography (EIT) that is similar to ECT in some respects. However, EIT is designed for scanning electrically conductive materials. This allows its use for scanning water-based fluids flowing in pipes as well as for medical imaging of some types of tissue. As previously discussed, ECT has been developed to scan relatively non-conducting materials such as oil-based fluids flowing in pipes. A discussion of the detection of rot in living trees using EIT has been reported by A. Borsic, Tomografia elettrica in bassa frequenza per il riconoscimento di anomalie in corpi cilindrici, Thesis for Laurea in Ingegneria Elettonica, Politecnico di Torino, 1998.
A variety of electrical imaging methods have been reported in the literature where measurements of transfer impedance using a system of electrodes external to a body are used to deduce the interior electrical properties (conductivity and permittivity). The earliest use is geophysical resistivity imaging, as reported for example by, Inman et al. Geophysics, 38:1088-1108 (1973) and by Dines and Lyle, Geophysics, 46 (7): 1025-1036 (1981). Application to imaging of the human body followed initiated by Hendersen, IEEE Biomed. Engr. Trans. 25 (3): 250-254 (1978), and realized in a practical form by Brown and Barber in U.S. Pat. No. 4,617,939 by employing the medical technique originally called Applied Potential Tomography APT, now mostly called Electrical Impedance tomography, EIT. Similar methods were later applied to imaging of industrial processes. Beck et al. used non-contact measurements of mutual capacitance to image permittivity of insulators. Later the Barber and Brown EIT method was applied to conductive media in process monitoring where the technique is called Electrical Resistance Tomography (ERT).
Given a system of N electrodes, reciprocity dictates that there are N(Nxe2x88x921)/2 independent measurements of mutual impedance that can be made. A complete set of measurements requires the application of a set of Nxe2x88x921 independent current patterns and the measurement of the voltages induced on the electrodes, or conversely the application of a set of Nxe2x88x921 independent voltage excitation patterns and the measurement on the corresponding currents. If the impedance of the medium is complex then the complex current and voltage must be known.
There are many possible measurement strategies. The excitation of current in pairs of electrodes in turn is usual in geophysical imaging, medical EIT and industrial ERT. Multiple current generators are used to drive optimal current patterns as described by Gisser""s U.S. Pat. No. 5,272,624, Isaacson""s U.S. Pat. No. 5,588,429 and as in the system OXBACT III described by Zhu et al. Physiological Measurements 15: (A37-A43)(1994). Multiple voltage drive systems have also been devised, for example the system discolosed by Kim, Clin. Phys. Physiological Measurements 8: 63-70 (1987), and the OXPACT II system described by Lidgey et al., Clinical Physics and Physiological Measurements, 13: 43-46 (1992). ECT systems typically use a voltage drive strategy where one electrode is excited with a given voltage and the current through the others measured to a virtual ground. The measurement can use a square wave excitation and charge transfer measurement circuit, or sine wave excitation as used in EIT. Medical EIT systems have been devised by Physiological Measurements 22:49-54 (2001) to employ multiple frequencies to characterize tissue properties.
White discloses an ECT device that can be used for detecting rot in wood in his 1996 dissertation titled Development of Phase-Sensitive Electrical Impedance Tomography for the Detection of Decay in Wood, University of Manchester Institute for Science and Technology, Manchester, England. White""s device employed the ECT method described above in which all non-sending electrodes are grounded and signal information is acquired from all non-sending electrodes simultaneously.
The White device employed non-contacting electrodes to measure electrical loss as a result of presence of decay in wooden power poles. Eight electrodes were strapped around pole circumference on a flexible belt. Current was measured by applying a radio frequency voltage at 500 KHz to a single electrode and measuring the current flowing to the other seven electrodes. The current was demodulated to measure magnitude and phase shift. A tomographic image of the decay pattern within the scanned cross sectional slice was then reconstructed.
White discussed the relative non-conductance of wood as his reason for adopting a typical ECT system approach. He employed large electrodes similar to those applied to relatively non-conducting fluids such as oil in pipes. The White device is similar to a typical ECT system in that the impedance of the signal transmitted between electrodes is measured. However, the White device measures both current magnitude and phase shift rather than only magnitude as Plaskowski et al. describe for typical ECT technology. White employs measuring only two-dimensional data in his described method. This approach may lead to significant errors in anomaly detection.
The White device differs from the invention in several respects. The White device differs from the TLDD in that the TLDD electrodes are in direct contact with the medium and those of the White device are non-contacting. The TLDD device may measure the voltage at each electrode of each possible electrode pair and may measure the impedance and/or current in both directions between each possible electrode pair. In contrast, the White device does not measure voltage at the receiving electrode. Because bi-directionality of transmission is an issue only for voltage in voltage out systems, White did not consider bi-directionality.
White also measures only a single signal frequency rather than employing multiple frequencies to improve anomaly detection as is employed by some embodiments of the TLDD. White employs only a single series of circumferential electrodes as opposed to the TLDD""s use in some embodiments of a multiplicity of circumferentially arranged electrodes for obtaining three dimensional data. White makes no provision for free-moving electrodes to accommodate a wood product of variable shape moving at speed between electrodes as is the case for the TLDD. White does not provide for use of electrodes of varying size and/or shape to improve their sensitivity to different anomaly sizes. Finally, White""s method is not amenable to application of the more accurate three-dimensional ECT methods while the motion of objects by the TLDD method will allow this application.
Oakley et al., Process Tomography-95: Implementation for Industrial Processes I(6-8):393-400 (1995), Paulson et al., Inverse Problems 11:425-437 (1995) and Isaksen, Measurement Science and Technology 7:325-337 (1996) disclose several ECT algorithms that allow reconstruction of a two-dimensional tomographic image based on sensed differences in material capacitance. Plaskowski et al. (1995) describe both ECT and EIT algorithms for reconstruction of two-dimensional tomographic images. Therefore, the algorithms and software techniques for reconstruction of images from data from multiple dielectric signals passed between pairs of electrodes are well documented. Some adaptation of these techniques is expected to suit them for application to the TLDD dielectric data.
Venter and Viljoen, in International Patent Number WO 96128741, titled Determining the Dielectric Properties of Wood describe a device for measuring the moisture content of a stack of lumber during kiln drying. Large electrode plates are placed across several pieces of lumber. In the example illustrated in the patent, a second lower electrode, of the same size and shape, is also placed across several courses of lumber but with seven layers of wood and six layers of air between it and the upper electrode. The lower electrode is grounded and radio frequency signal is applied to the upper electrode. The electric signal passes through the alternate layers of wood and air to the lower electrode and the magnitude and phase shift of the signal are measured as the wood is dried by the heat in the dry kiln.
The circuit to accomplish this application includes a signal generator connected to a load driver that imposes the radio frequency signal to a resistor. The load driver is required to allow the radio frequency signal to be driven across the high resistance created by the multiple layers of wood and air in the drying lumber. The Venter and Viljoen device measures moisture changes in the lumber between the electrodes as related to the difference in the input voltage denoted v1 and the voltage transmitted through the lumber and air courses which is denoted v2. This difference allows a direct determination of the increasing magnitude and an indirect computation of the phase shift that occurs as wood moisture content decreases during kiln drying. These voltage v1 is measured prior to the resistor and v2 just after the resistor.
The only variable wood characteristic that the Venter and Viljoen device is described as detecting is moisture content. No application of the device to determine density differences to detect anomaly presence is reported.
Steele and Cooper have disclosed a patent titled Moisture and Density Detector (Pending) for sensing moisture and/or density differences in wood or other dielectric materials. The Moisture and Density Detector (MDD) circuits measure dielectric values of magnitude and phase shift. The potential use of the MDD to detect wood types is disclosed, but detection of localized anomalies such as knots, voids, decay, etc. are not. In addition, the MDD electrodes are directly opposed to scan through wood thickness or are directly adjacent for scanning horizontally across wood surface. No disclosure of a circular electrode arrangement to detect anomalies in logs, poles, trees, or thick materials such as cants or timbers is made. The Steele and Cooper device also employs only voltage magnitude and phase shift measurements but not impedance magnitude and phase shift. No consideration is given to bi-directional electrode query to allow better detection of anomaly position between electrode pairs. The MDD directly opposed or directly adjacent electrode positioning is intended to allow scanning through relatively flat objects such as lumber or a thin flat layer of dielectric materials such as chips carried on a conveyor.
An impedance detector disclosed by Tiitta et al., Development of an Electrical Impedance Spectrometer for the Analysis of Wood Transverse Moisture Gradient, Proceedings of the 12th International Symposium on Non-Destructive Testing of Wood, September 13-15, University of Western Hungary (2000), measures the moisture gradient in wood. Tiitta et al. termed this method spectral impedance. By this method, electrodes contained in a probe were placed on the wood surface. One electrode transmitted an electrical signal at frequencies below 5 MHz, and the second received the signal. A variable electric field was developed between the electrodes. Analysis of the behavior of impedance for the various frequencies transmitted through the wood allowed estimation of the moisture gradient within the wood. This technique of spectral impedance has not been used to detect anomalies in wood.
Sobue, Measurement of Moisture Gradient in Wood by Electron Scanning Moisture Analysis (ESMA), Proceedings of the 12th International Symposium on Non-Destructive Testing of Wood, September 13-15, University of Western Hungary (2000) discloses paired electrodes in the radio frequency range to detect the moisture gradient in wood. Rather than vary frequency and analyze the impedance spectrum as did Tiitta et al., Sobue varied the distance between the electrodes. This distance variance modified the capacitance of the signal. Sobue analyzed the signal behavior for the several capacitances produced and estimated moisture gradient based on this analysis. Sobue did not employ this device to detect anomalies in wood.
A need, therefore, exists for devices and methods for detecting anomalies as evidenced as locations of differential density in logs, cants, timbers, poles or trees using dielectric signals passed through the wood comprising these products.
It is an object of the present invention to provide a method and apparatus for determining the density of logs, cants, timbers, poles, or trees.
It is another object of the present invention to provide a method and apparatus for detecting localized anomalies, such as knots, in logs, cants, timbers, poles, or trees.
These and other objects of the invention are achieved by a method and apparatus for detecting areas of differential density in logs, cants, timbers, poles, or trees and the like to determine anomalous internal characteristics such as knots, voids, decay, and wood type. The method comprises applying signals to one electrode attached to the wood piece and measuring the output at a plurality of other electrodes in contact with the wood piece. Preferably, the transmitted signal is applied to each electrode of an electrode pair (sometimes referred to herein as bi-directional electrode query) in order to detect smaller anomalies such as knots. The signal transmitted to the sending electrode is preferably a radio frequency signal. The signal received at the receiving electrode may be measured as voltage, impedance or current. The magnitude and/or phase of each signal type may be measured. The data from combinations of all possible pairs of electrodes is analyzed to determine a location of an anomaly. Location, depth and area of anomaly may be estimated. Alternatively, tomographic techniques, such as those currently employed for EIT, may be applied to develop an image of density differences in the scanned cross-sectional data. Such techniques are well known in the art and will not be discussed in further detail herein.
One embodiment of the invention comprises multiple pairs of electrodes disposed at predetermined angles to each other. Electrodes may be of any size or shape and are in direct contact with wood surface. Electrodes may move freely to accommodate the differentiated shape of the wood product passed between them. Adjustments are made for electrode displacement as electrodes move with respect to each other.
Measurements at electrodes may be made sequentially or simultaneously. When measuring voltage or current at a particular electrode, or when measuring impedance between a pair of electrodes, electrodes other than the electrode to which the input signal is applied and the particular electrode at which the measurement is being taken, or other than the electrodes between which impedance is being measured, may be grounded or ungrounded. When measuring current at an electrode, that electrode should be grounded. Determination of both magnitude and phase shift of the voltage, impedance, and current signals may be performed to provide additional information. Any circuit allowing measurement of current, voltage and impedance may be utilized for the apparatus.
The above and other objects and advantages of the present invention will become more apparent from a reading of the following detailed description of the invention in conjunction with the drawings and the appended claims.